The Role of BiP Nucleotide Exchange Factor Sil1 in Immunoglobulin Biosynthesis
BiP is an essential endoplasmic reticulum resident molecular chaperone of the HSP70 family that binds exposed hydrophobic regions of unfolded proteins. Substrates bound by BiP are protected from dangerous non-specific interaction with other unfolded proteins via their aggregation prone exposed hydrophobic regions, which are normally buried in the native state, and act as BiP binding targets. For these substrates to mature into the native state, BiP must be released. Like all HSP70 family members, cycles of BiP binding and release are nucleotide dependent and facilitated by two genres of cochaperones, HSP40s, and nucleotide exchange factors (NEFs). HSP40s function in stimulating the hydrolysis of ATP, which causes a conformational change that induces tight binding of BiP to substrate, whereas, NEFs stimulate the escape of ADP that allows BiP to release substrate so that it can progress toward its final native state. In 2002, we identified a mammalian BiP associated protein, Sil1, with nucleotide exchange activity. In 2005, Sil1 became the first molecular cochaperone known to cause disease upon discovery of mutations that lead to Marinesco-Sjögren Syndrome (MSS), a debilitating recessive disease characterized by severe cerebellar atrophy and a wide range of systemic defects. Now, many Sil1 mutations in MSS are known to result in interrupted BiP binding or Sil1 instability. We have attained an animal model of MSS with a Sil1 mutation that results in loss of a major BiP interaction site. This model, known as the Woozy mouse due to its striking ataxic phenotype, closely phenocopies many aspects of human MSS yet has been the spotlight of very few studies. For the first time, we have performed a detailed examination on the effects of Sil1 loss on a specific BiP substrate, immunoglobulin (Ig) heavy chain. This is a highly relevant target to study for two main reasons. First, because Ig is the best characterized BiP substrate and is readily tractable by several well-established methods. Second, because production of Ig is an essential part of primary and secondary immune responses that have not been examined in MSS patients, or anywhere in terms of Sil1 function. We examined levels of total IgM, IgG2b and IgG1 as well as antigen-specific levels of these same antibodies in the serum of woozy mice by ELISA before and after immunization. We discovered significantly lower levels of total IgM in Woozy mice but, surprisingly, did not observe any defects in total production of other Ig isotypes. We were also surprised to find no differences in concentration of antigen-specific Ig in serum after immunization, including IgM levels. Neither did we observe any defects in IgM production by western blotting or ELISA of cell supernatant from cultured primary B cells subjected to LPS stimulation. Furthermore, although we hypothesized that Sil1 null cells may have higher levels of some chaperones or cofactors as part of an ER stress induced compensatory mechanism for Sil1 loss (this has been shown to be the case in Purkinje cells of the neurodegeneration prone cerebellum in Sil1 KO mice), this was not our observation in B cells or activated plasmablasts. Based on our data from Woozy mice and also from primary cultured B cells, we hypothesize that individuals affected with MSS will produce normal levels of Ig in response to antigen, which is necessary for healthy primary and secondary immune responses. We also have additional preliminary data to suggest that there may be some age related defects in the Woozy mouse model that have not been previously reported. However, these studies are ongoing and solicit further investigation.
BiP, Sil1, Grp170, MSS, Marinesco-Sjögren Syndrome, Nucleotide exchage factor, Medical Cell Biology, Medical Immunology, Medical Sciences, Medicine and Health Sciences
University of Tennessee Health Science Center
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